Modern diesel engines are run by computers, and a fault code is where diagnosis starts — not where it ends.
When something goes wrong, the engine control module logs a fault and, depending on how serious the problem is, lights up the dash, limits power, or shuts the engine down. A NOx sensor code could be a bad sensor, a wiring problem, a DEF quality issue, or an exhaust leak upstream. Replacing parts based on the code alone is how trucks end up with three new sensors and the same problem.
Mobile diesel diagnostics means a mechanic comes to your truck with a scan tool and diagnostic software, reads active and inactive fault codes, looks at live sensor data, and tests the components involved. The goal is to fix the actual problem the first time.
For a driver on the shoulder, the most important answer is often the simplest one: is this safe to drive to the destination, or does it need to be fixed right here?
What our Diesel Engine Diagnostics covers
- Reading active and inactive fault codes on engine, transmission, ABS, and aftertreatment modules
- Reviewing live data — boost, fuel pressure, coolant and oil temperature, exhaust temperatures, and DPF differential pressure
- Derate and limp-mode troubleshooting
- Sensor testing and replacement: NOx, exhaust temperature, DPF differential pressure, coolant temperature, oil pressure, and crank and cam position
- Wiring and connector inspection, including chafed harnesses and corroded pins
- Checking for boost leaks, charge air cooler leaks, and cracked intake piping
- EGR valve and EGR-related faults
- Turbocharger actuator faults
- Initiating parked regens and other service routines when the software and system allow
- Clearing codes after a repair and confirming the fault doesn’t come back
- A plain-English explanation of findings so a fleet manager or shop knows exactly what was found
What can't be done on the roadside
Diagnostics can pinpoint almost any problem, but not every problem found can be repaired on site:
- Internal engine damage — low compression, a failed injector cup, or coolant in the oil — usually means shop work
- Some module reprogramming and parameter changes require OEM dealer access
- Warranty-covered failures may need to go to an authorized dealer to keep coverage
- Repairs that need extended engine teardown, like EGR cooler replacement on some engines, are often better done in a shop
Even when the repair has to happen elsewhere, a clear diagnosis saves money: the truck goes to the right shop, for the right job, with parts ordered ahead of time.
Fix it on site or tow it?
A code doesn’t automatically mean a tow. Many faults come down to sensors, wiring, or connections that can be repaired where the truck sits. Some faults let you drive in reduced-power mode to a safe location or a shop. Others — low oil pressure, high coolant temperature, or anything that shuts the engine down — mean the truck shouldn’t be driven until the cause is found.
A red stop-engine light means exactly what it says: get off the road safely and shut the engine down. An amber warning usually gives you more room, but keep an eye on it and call before it escalates.
What to tell dispatch when you call
The more dispatch knows up front, the better the odds the mechanic shows up with the right parts and tools on the first trip.
- Which lights are on (check engine, stop engine, DPF, DEF, high exhaust temperature) and any message on the dash
- Fault codes, if the dash displays them
- Whether the truck is derated, and how much power it has
- Engine make and model — Cummins X15, Detroit DD15, PACCAR MX-13, Volvo D13, and so on
- What the truck was doing when the light came on — pulling a heavy load, idling, just after fueling
Truck makes & engines we work on
- Freightliner
- Peterbilt
- Kenworth
- Volvo
- Mack
- International
- Western Star
Class 6 through Class 8 trucks, including engines from Cummins, Detroit Diesel, PACCAR, Volvo, Mack, Navistar, and Caterpillar.
Diesel Engine Diagnostics around the harbor & the South Bay
Diagnostics calls come from everywhere a truck can park: container yards along Harry Bridges and Anaheim, warehouse docks in Carson and Rancho Dominguez, and the shoulders of the 710, the 110, and the 405 where a derate forced the driver to pull over. A fair number also come from Terminal Island, where a fault light means finding somewhere to stop before you can even start diagnosing it.
Freeways
Service areas
Diesel Engine Diagnostics FAQ
Can you tell me if my truck is safe to drive?
That’s usually the first question we answer. Some faults allow the truck to be driven normally or in reduced-power mode; others mean the engine should stay off until it’s repaired.
Can’t I just clear the code and keep going?
Clearing a code doesn’t fix the cause, and many emissions faults will come back or escalate to a harder derate. Some faults won’t clear at all until the problem is repaired.
What does a derate mean?
The engine computer has limited power or speed, either to protect the engine or to force an emissions repair. Derates often get worse in stages if the problem isn’t fixed.
Do you have software for my engine?
Tell dispatch your engine make and model when you call so the mechanic comes prepared for your truck.
Can you diagnose a problem that comes and goes?
Intermittent problems are harder, but inactive fault codes and freeze-frame data often show what was happening when the fault occurred. Describe when it happens as precisely as you can.
Can you check transmission and ABS lights too?
Yes. Transmission and ABS modules store fault codes too, and they get read along with engine faults.